The present invention relates to image forming equipment of the type forming an image by an electrophotographic procedure and, more particularly, to a device incorporated in this type of equipment for controlling the cleaning of an image carrier implemented with a photoconductive element, i.e., the removal of a toner remaining on the image carrier.
An electrophotographic copier, facsimile transceiver and laser beam printer are typical examples of image forming equipment of the type forming an image by executing an electrophotographic procedure that includes a charging, exposing, developing, transferring, separating, and cleaning steps, as is well known in the art. This type of image forming equipment has a photoconductive element which plays the role of an image carrier for carrying an image thereon. To remove toner which remains on the photoconductive element after image transfer, the equipment executes a cleaning step by use of a cleaning roller which is applied with a cleaning bias voltage and on which a cleaning agent forms a magnet brush. To enhance the cleaning ability, it has been customary to charge, before the above cleaning step, the photoconductive element to the same polarity as the remaining toner to thereby reduce the electrostatic cohesion acting therebetween. This is successful in facilitating the removal of the remaining toner from the photoconductive element and is usually referred to as precleaning discharge. The cleaning ability achievable with the magnet brush formed on the cleaning roller is noticeably effected by the electric field which is developed at the time of cleaning, i.e., the potential on the photoconductive element undergone precleaning charge and the bias voltage applied to the cleaning roller. Hence, a desirable cleaning characteristic is not attainable unless the value of such an electric field for cleaning is confined to an adequate range. Should the electric field have a value not lying in the adequate range, the toner would again deposit on the photoconductive element after the cleaning step, degrading the cleaning ability. It has been reported that the adequate electric field range depends on the amount of toner remaining on the photoconductive element. In light of this, the bias voltage applied to the cleaning roller may be controlled on the basis of a detected amount of toner remaining on the photoconductive element, as disclosed in Japanese Patent Laid-Open Publication No. 176085/1982 by way of example. This kind of scheme, however, cannot compensate for the deterioration of the cleaning agent that forms a magnet brush on the cleaning roller, i.e., the change in the resistance of the agent, failing to confine the electric field in the adequate range with accuracy. It is, therefore, impracticable to enhance the cleaning ability and stabilize it over a long period of time.